Activity-dependent plasticity of developing climbing fiber-Purkinje cell synapses.

Elimination of redundant synapses and strengthening of the surviving ones are crucial steps in the development of the nervous system. Both processes can be readily followed at the climbing fiber to Purkinje cell synapse in the cerebellum. Shortly after birth, around five equally strong climbing fiber synapses are established. Subsequently, one of these five synaptic connections starts to grow in size and synaptic strength, while the others degenerate and eventually disappear. Both the elimination of the redundant climbing fiber synapses and the strengthening of the surviving one depend on a combination of a genetically coded blueprint and synaptic activity. Recently, it has been shown that synaptic activity affects the synaptic strength of developing climbing fibers. Remarkably, the same pattern of paired activity of the presynaptic climbing fiber and the postsynaptic Purkinje cell resulted in strengthening of already "large" climbing fibers and weakening of already "weak" climbing fibers. In this review, we will integrate the current knowledge of synaptic plasticity of climbing fibers with that of other processes affecting climbing fiber development.